This is basic metallurgy deriving from the Al-Cu and Al-Si phase diagrams. Aluminum has significant solid solubility for Cu, 5.67 wt%, but very little for Si, 1.65 %, at the respective Al-rich eutectics. Hence, the aluminum matrix is solid solution hardened by Cu. It is further hardened by a finely dispersed CuAl2 precipitation within the Al grains, which contributes to a free machining effect.
In Al-Si alloys, the Si is essentially a pure phase precipitated within a relatively pure, hence softer, Al matrix. During machining, large, hard Si grains can be dragged along the surface, i.e., smearing. This is one reason for using grain refiners in Al-Si alloys.
The machinability of Al alloys is considerably more complex than in the brief description above. Pure binary alloys are never used (except in research); commercial alloys contain both intentional and impurity alloying elements. Machinability is affected by a host of alloying effects, solidification rates, heat treatment, cold working, tooling variables, etc., etc.
Among wrought alloys, the relatively pure, soft alloys 1100, 1350 and 5005 in annealed temper are rated worst for machining due to continuous, stringy cutting chips. Their machinability can be improved by cold working (hardening). The Cu-containing 2xxx alloys in age-hardened tempers are considered best.
Among casting alloys, Si is present in highest concentration (to 22%) in the 3xx alloys. The hard Si grains cause cutting tool wear and poor cutting. Both increasing silicon particle size (at constant Si content) and increasing Si content (at constant particle size) increase tool wear. In both hypo- and hypereutectic alloys, adding a grain refiner creates smaller but more numerous Si particles, reducing tooling wear. Grain refinement is most important for slower solidification rates.
For both wrought and cast alloys, coarse solidification phases are detrimental to machining. Solutionizing dissolves phases containing Cu or Mg, and age-hardening is beneficial, as the extremely minute precipitates improve machinability.
Small additions of bismuth, cadmium, lead and tin have traditionally been used to make free-machining alloys. Cadmium and lead are being phased out (I doubt Cd was ever used much); indium and tin are used in combination in some newer alloys.
Some aluminum reference books include Forming and Machining Aluminum Alloys, among some 145 titles available from the Aluminum Association
kenvlach failed to mention one thing that is very important in machining Aluminum and that is the cutting fluids used. Always use a cutting fluid made for use with Al. We never found a universal cutting fluid that was good for Al.
We constantly machined 356 Al 24"-30" disks that had to have finish 16 RMS or better. After trying hundreds of cutting fluids we tried one that had been used for difficult machining jobs for years and got the best finish we had ever gotten, a 10-12 RMS.
Don't laugh, the cutting fluid was Kerosene and Lard (pig fat).
